1. Field of the Invention:
The invention is directed to a method and an apparatus for deriving an off-course signal for an orbiting earth satellite by means of an earth-horizon sensor which operates in the infrared range and includes, in the focal plane of an input lens, a chopper disk which is periodically reciprocated at a determined chopper frequency and has a diameter corresponding to an image of the earth, and a detector for receiving periodically interrupted light radiations from opposite earth rims, wherein the output signal of the detector is demodulated synchronously with the chopper frequency and is corrected with a correcting signal (anomaly signal) corresponding to different temperatures of the opposite earth rims (earth anomaly) in order to form an angle-dependent off-course signal.
2. Description of Related Art:
For the attitude control, preferably triaxial of such triaxial, geostationary earth satellites off-course signals are required which indicate the deviation of a line of sight in relation to the satellite/earth central point connecting line, the line of sight being fixed relative to the satellite. For this purpose, optical earth-horizon sensors operating in the infrared range, which can be classed with ZERO-seeking sensors, are used, among others, for two axes. Such an earth-horizon sensor is based on the mechanical vibrator or chopper principle. The infrared radiation of the earth is collected through an input lens of germanium and falls on a circular chopper disk in the focal plane of this input lens. The chopper disk has a diameter approximately corresponding to the image of the earth and is periodically reciprocated with a determined frequency, the chopper frequency, and a determined amplitude, the chopper amplitude. The light from the two opposite earth rims, which is alternately released with the chopper frequency, i.e. the light falling through the input lens and being periodically interrupted by the chopper disk, is directed on a detector, e.g. a pyroelectric detector, via a spectral filter for the infrared range, a secondary lens consisting of a spherical mirror segment and a prism.
The output signal of the detector is amplified and subsequently synchronously demodulated with the chopper frequency. If the radiation energy received by the detector from the two earth rims is equal, a ZERO-signal is supplied by means of the synchronous demodulation. In this case, the line of sight of the earth-horizon sensor, which line of sight is fixed relative to the satellite, is directed directly at the earth central point, i.e. the satellite is located in the desired position. When the satellite is in an incorrect position, the line of sight of the earth-horizon sensor no longer coincides with the satellite/earth central point connecting line so that the output signal of the detector depends on the difference of the received radiation energy from the opposite earth rims. After the demodulation of the output signal synchronously with the chopper frequency, an off-course signal corresponding to the difference is obtained in this way which is a direct measurement of the off-course angle of the line of sight of the earth-horizon sensor in relation to the satellite/earth central point connecting line.
The off-course angle can be given in one axis with such an earth-horizon sensor. For triaxial stabilization, two such earth-horizon sensors are needed with which the off-course angle of the earth satellite is given, e.g. in the north-south and east-west direction.
The chopper disk is periodically reciprocated with a chopper frequency of approximately 40 Hz and with an amplitude of approximately 1/12 of the diameter of the chopper disk, i.e. by approximately 1.4 mm.
However, when determining the off-course angle it must be taken into account that the opposite earth rims do not always have the same temperature. This fact is designated as earth anomaly. The earth anomaly is particularly conspicuous in the north-south direction. This earth anomaly causes the off-course signal derived as described above to be affected with an anomaly error which can be regarded essentially as a parallel displacement of the characteristic line of the earth-horizon sensor. This anomaly error is, accordingly, essentially a zero point error and although it is only slight, it can, in extreme cases, amount to approximately .+-.20% of the linear measuring range of approximately one degree of the earth-horizon sensor.
Since geostationary earth satellites are also used for producing directional radio links and guided television links to the earth, the off-course signal which is subject to error because of the earth anomaly must be corrected bv the anomaly portion. A plurality of methods are known for this purpose which are described in German Patent Application No. P 33 22 750.0 of the Applicant which is now copending application Ser. No. 745,020. Reference is made to this patent application. The idea of correcting the off-course signal is based substantially on the knowledge that the anomaly portion of the off-course signal has a very particular relationship to the respective chopper amplitude. By changing the chopper amplitude, the anomaly portion of the off-course signal can be calculated and the off-course signal can be corrected. Thus, for example, the chopper amplitude can be modulated or intermittently altered, or two earth-horizon sensors with different chopper amplitudes can be used. In small amplitude variations, the anomaly signal can be described by means of a horizontal straight line. In larger amplitude changes, the anomaly signal has a non-linearity which is dependent on the off-course angle, since the characteristic lines of the earth-horizon sensor do not run parallel for various amplitudes. In a narrowed measuring range of approximately .+-.1.degree., the anomaly signal in the known amplitude variations can be taken into account by means of a signal which is linearly dependent on the off-course angle. The above-mentioned patent application is referred to for taking into account the anomaly signal and for the correction of the off-course signal accordingly made possible.